I think Butt Rock was coined by older Gen-Xers with a definition like that given above by the writer "Reformed Butt Rocker": Butt Rock is plodding 1970s boogie rock, music of the sort that the characters in the movie "Dazed and Confused" might have listened to. For the people born a decade later, the term came to mean the working class rock of the 1980s--this being the heavy metal that was mainstream enough to get played on MTV in prime time. The common denominator is that the term derides the music that is associated with lower-class stoners, football hooligans and their American equivalents, and other uncouth types. The original definition may be antiquated, but it's still valuable knowledge, linguistically speaking.
Foghat's "Slow Ride" - the Platonic ideal of a Butt Rock song, released during the (original) Butt-Rock peak of 1975.